Converters of various designs operated as rectifiers may be used for feeding direct current systems out of three-phase current sources, in particular of motor vehicle electrical systems using three-phase current generators. Converters having a six-, eight-, or ten-pulse design are generally used in motor vehicle electrical systems, corresponding to the three-, four-, or five-phase current generators which are usually installed. However, the present invention is also suitable for converters having other numbers of phases or pulses.
When reference is made below to a generator for the purpose of simplicity, this can also be an electric machine which is operable in a generator mode and a motor mode, for example a so-called starter generator. In the following discussion, a converter is understood to mean a bridge circuit of a known type, which operates as a rectifier in a generator mode of the electric machine. For simplicity, reference is also made below to a rectifier. A system made up of an electric machine that is at least operable in generator mode and a corresponding converter that operates as a rectifier is also referred to below as a current supply device.
So-called load shedding (dumping) is a critical operating condition for such current supply devices. Load shedding occurs when, for a highly excited electric machine and a correspondingly high delivered current, the load on the electric machine or the converter suddenly decreases. Load shedding may result from a disconnection of consumers in the connected motor vehicle electrical system, or from a cable break.
When consumers are suddenly disconnected in a motor vehicle electrical system, in particular during battery-free operation, due to the inductance of the excitation winding and the excitation field, which therefore decreases only slowly, for up to one second the electric machine may supply more energy than the motor vehicle electrical system is able to receive. If it is not possible to intercept or completely intercept this energy by capacitive elements in the motor vehicle electrical system or in the converter, overvoltages and overvoltage damage to components in the motor vehicle electrical system may occur.
In the event of a cable break, as a result of which the motor vehicle electrical system is disconnected from the converter, the electric machine likewise continues to supply energy, but a consumer is no longer connected. In comparison to the case just discussed for the disconnection of consumers, the consumers are thus no longer endangered. The consumers can also continue to be supplied by the battery. However, as the result of overvoltages, in such cases the power electronics of the electric machine or of the converter may be damaged.
In conventional (passive) converters, in each case a certain amount of protection of the vehicle electrical system or the power electronics of the electric machine and of the converter is provided by the converter itself, namely, with the aid of the Zener diodes, installed there in the classical case, in which the overvoltage is arrested and the excess energy is converted into heat. The use of additional arresting elements is also known in this regard.
However, the use of active or controlled bridge converters is desirable in motor vehicles since, among other reasons, active converters, in contrast to passive or uncontrolled converters, have lower power losses during normal operation.